The invention relates to a method of manufacturing biconvex lens elements comprising a biconvex glass lens and a metal holder, in which a lens blank having an overdimensioned glass volume is heated together with a metal holder to the processing temperature of the glass, the holder with the lens blank is arranged between the heated dies of a mould, which dies have at their mould surfaces a concave profile corresponding to the convex profiles of the glasslens to be formed, the two dies are then moved towards each other in a pressing stroke, the lens blank is moulded in the holder into a biconvex glasslens, an excess quantity of glass is pressed away during moulding and the lens element thus obtained comprising a glass lens and a holder is cooled.
Such a method is known from U.S. Pat. No. 4,398,935. By this method, biconvex lenses can be manufactured, which satisfy high requirements with regard to accuracy to shape and size without further aftertreatments. In this known method, weight variations of the lens blank can be neutralized in that the excess quantity of glass is pressed away through a radial gap between the dies and forms a radial flange on the moulded product after cooling. A biconvex lens with such a radial flange is known, for example, from U.S. Pat. No. 4,391,622. If the glass in the radial gap is cooled prematurely and solidifies, the solidified glass counteracts a build-up of pressure in the mass of glass due to the fact that the dies run against the solidified glass and a further pressurizing of the glass becomes impossible. The build-up of pressure is not optimal.
The processing temperature is that temperature at which the glass has such a viscosity that the glass moulding processes can be carried out in the desired manner. The optical glasses suitable for use are processed at a viscosity of 10.sup.2.5 to 10.sup.5.0 Pa.s (10.sup.3.5 -10.sup.6.0 poises).